Expose the AsciiDoc counter facility as an API

[mojavelinux]

Asciidocotor has built-in support for counter attributes. Provide an API to allow the counter under these attributes to be used. This is particularly useful in extensions.

This method should be added to Document:

• counter(String name, Object seed) - Get the next number in the sequence for this counter, initializing it with the seed value if not already set

We should verify that after a counter is retrieved from the API, it continues to get used when resolved in the AsciiDoc source.

{counter:tab-index}

There are methods on the Ruby API that can be used to control the internal state.

[robertpanzer]

@mojavelinux Would be great if you could take a look at #493